Real-Time Control for 42-m Telescope

What’s 42 meters in diameter and needs 3,000 actuators? The answer: The world’s largest telescope, the European Extremely Large Telescope (E-ELT), which is now being planned by an organization called the European Southern Observatory.

Construction on the telescope is expected to begin sometime next year and take seven years to complete. One of the advanced concepts it embodies is the use of adaptive optics to compensate for atmospheric effects and to keep images collimated.

The E-ELT will contain five mirrors. The primary 42-m mirror will actually consist of 984 hexagonal mirrors, each weighing about 330 lb with diameters between 1.5 and 2 m. The hexagonal elements will be tilted under computer control with feedback loops built around 3,000 piezoelectric actuators and 6,000 edge sensors.

As part of a feasibility study, computer control is being simulated on two Dell Precision T7400 workstations each running LabView Real- Time Modules from National Instruments Corp., Austin, Tex. For each hexagonal element, the software computes a 6,000-vector product array from values corresponding to actuators and sensors. Computations take place at 1-kHz rate. Right now, the computers and software are running hardware-in-the-loop testing.

In the real system, outputs would go to actuators to adjust the mirror.

One of the other four mirrors in the telescope is a 2.5-m in diameter, thin and deformable mirror that helps generate an exit beam for observers. The smaller mirror changes shape to correct for wavefront distortions that arise because of atmospheric effects. This mirror will incorporate 5,000 piezoelectric actuators and several thousand wavefront sensors. It is part of a configuration of mirrors designed to create a focal plane that is unaberrated regardless of where observers choose to do the viewing.

Currently, the control computations for the smaller mirror run on a 16-blade Dell system with each blade containing eight cores. Control of this mirror represents four 5,000 × 14,000 matrix operations that must get computed at 500 Hz. Another LabView Real-Time module handles this task by dividing computational chores among 128 cores.

The computers involved in primary mirror control also use a time-triggered network feature within the Real-Time modules to exchange data between the control system and the mirror simulator. This should eliminate the non-deterministic qualities of ordinary Ethernet links. The resulting network moves 36 Mbytes/sec in a deterministic way.